Symbols sqlite: fix sourcegraph/server build
Created by: chrismwendt
I broke the sourcegraph/server:insiders Docker image in https://github.com/sourcegraph/sourcegraph/pull/2334 by forgetting to build libsqlite3-pcre and set LIBSQLITE3_PCRE when building sourcegraph/server.
This factors out the libsqlite3-pcre part of cmd/symbols/Dockerfile as cmd/symbols/libsqlite3-pcre/Dockerfile so that it can be used in cmd/server/Dockerfile.
This is a continuation of: